1. Field of the Invention
The present invention relates to a hydraulic-pressure control system for a four-wheel drive vehicle equipped with a hydraulic-pressure operated transfer clutch, and specifically to techniques for controlling a hydraulic pressure supplied to a transfer clutch for the purpose of optimally adjusting a driving-torque distribution between front and rear drive wheels.
2. Description of the Prior Art
Recently, there have been proposed and developed various driving-torque distribution control system for four-wheel drive vehicles in which a part of driving torque can be delivered from primary or main drive wheels, such as rear drive wheels to secondary or auxiliary drive wheels, such as front drive wheels by means of a transfer, depending on the difference of revolution speeds between front and rear drive wheels. One such driving-torque distribution control system has been disclosed in Japanese Patent Provisional Publication (Tokkai Heisei) No. 2-270641. In the Japanese Patent Provisional Publication No. 2-270641, the driving-torque control system is exemplified in case of a four-wheel drive vehicle in which rear drive wheels serve as primary drive wheels whereas front drive wheels serve as secondary drive wheels. In the above-noted prior art system, the greater the difference of revolution speeds between rear and front drive wheels, (i.e., the greater the slip ratio of the rear drive wheels), the greater the engaging force of the transfer clutch, so as to increase the driving-torque distribution ratio of the front drive wheels to the rear drive wheels, thus effectively and rapidly suppressing wheel slip (often called acceleration slip) at the rear drive wheels (the primary drive wheels). The prior art driving-torque control system utilizes a hydraulic circuit as shown in FIG. 11, for controlling the engaging force of the transfer clutch.
Referring now to FIG. 11, working fluid (hydraulic oil) in an oil reservoir 1 is pressurized by way of a hydraulic pump 2. The pressurized working fluid is supplied a supply line of the hydraulic circuit as a line pressure PL. The line pressure PL is decreasingly regulated at a predetermined pressure level by means of a line-pressure regulation valve 3, and fed to a clutch-pressure control valve 4. The clutch-pressure control valve 4 is hydraulically connected to a duty-ratio controlled electromagnetic solenoid valve 5, for decreasingly adjusting the incoming line pressure PL in response to a controlled pressure output from the duty-ratio controlled solenoid valve 5 and based on a duty ratio imparted to the duty-ratio controlled solenoid valve 5, and for outputting a controlled clutch pressure PC. The controlled clutch pressure PC is supplied via a pilot-operated directional control valve 6 to the transfer clutch 7. The pilot-operated directional control valve 6 is hydraulically connected to an electromagnetic directional control valve 8, such that the valve position of the valve 6 is selectively switched between two positions depending on the presence or absence of the external pilot pressure from the electromagnetic directional control valve 8, so as to enable or disable the supply of the clutch-pressure to the transfer clutch 7. If the valve 8 is de-energized, in the absence of the external pilot pressure from the valve 8, the fluid communication between the inlet port of the valve 6 and the inlet port of the transfer clutch 7 is blocked and also the inlet port of the valve 6 is communicated with a drain port connected to the reservoir, and as a result the transfer clutch 7 is released and thus the vehicle is operated at the two-wheel drive mode (the rear-wheel drive mode). If the electromagnetic directional control valve 8 is energized, and thus the external pilot pressure is output from the valve 8 to the valve 6, a full fluid communication between the inlet port of the valve 6 and the inlet port of the transfer clutch 7 is established. In this case, the higher the clutch pressure PC, the greater the engaging force of the transfer clutch 7, and thereby increase the driving-torque distribution ratio of the secondary drive wheels (the front drive wheels) with respect to the primary drive wheels (the rear drive wheels). On the other hand, the duty-ratio controlled solenoid valve 5 is associated with the clutch-pressure control valve 4, so that the clutch pressure PC is decreasingly adjusted in accordance with the increase in the duty ratio applied to the duty-ratio controlled solenoid valve 5. For instance, with the duty ratio held at 0%, the clutch-pressure control valve 4 is maintained at its full-open position, thus permitting the incoming line pressure PL to be output from the clutch-pressure control valve 4 as a maximum secondary pressure (the maximum clutch pressure). In case of selection of a two-wheel drive range or mode, the electromagnetic directional control valve 8 and the duty-ratio controlled solenoid valve 5 are maintained at their de-energized positions, thus shutting off the flow of working fluid of the maximum clutch pressure from the clutch-pressure control valve 4 to the transfer clutch 7 by means of the pilot-operated directional control valve 6. In the prior art system as disclosed in the Japanese Patent Provisional Publication No. 2-270641, the clutch pressure PC in the outlet port of the clutch pressure control valve 4 can be maintained at a high pressure level in the case of the two-wheel drive mode, and thus the prior art system can quickly switch from the two-wheel drive mode to the four-wheel drive mode with a high response. Also, the conventional system can provide such a fail-safe function that the clutch pressure PC can be reliably fed from the clutch pressure control valve 4 to the pilot-operated directional control valve 6, even in case of breaking of a signal line through which a duty-cycle controlled exciting current is supplied to the solenoid of the duty-ratio controlled solenoid valve 5. Also in case of a four-wheel drive vehicle which can operate at a selected one of three modes, namely a four-wheel-drive low-speed range (4L) in which the vehicle is held forcibly at a four-wheel drive state by way of a mechanical locking means, a four-wheel-drive high-speed range (4H) in which engine power is properly distributed between front and rear drive wheels via a transfer clutch, and a two-wheel-drive high-speed range (2H) as previously explained, when the four-wheel-drive low-speed range is selected, the electromagnetic directional control valve 8 and the duty-ratio controlled solenoid valve 5 are both de-energized, and thus the supply of the clutch pressure PC is stopped in the same manner as the two-wheel-drive range. In this case, the switching operation from the two-wheel-drive range 2H or the four-wheel-drive low-speed range 4L to the four-wheel-drive high-speed range 4H can be quickly achieved.
Each of the above-noted pilot-operated directional control valve 6 and the clutch-pressure control valve 4 traditionally comprises a spool valve that slidably accommodates a spool in a cylindrical valve housing. The spool is biased in a normal position by way of a return spring. The position of the spool is controlled by a controlled pressure acting on a pressure receiving surface of the spool in the opposite direction to the direction of action of the spring bias, so as to produce a properly regulated secondary pressure. The prior art system suffers from the drawback, undesirable oil leakage via an annular aperture defined between the cylindrical sliding surface of a land of the spool and the inner wall surface of the valve housing, in presence of a great pressure difference between oil passages upstream of and downstream of the annular aperture. For example, in case of the pilot-pressure operated directional control valve 6, in the event that the electromagnetic directional control valve 8 is de-activated, the spool of the valve 6 is maintained at the spring-biased position (corresponding to the normal position) as seen in FIG. 12. In FIGS. 12 and 13, the broken lines indicate a return spring. In FIGS. 11, 12, and 13, X denotes a drain port. Under this condition, even though the inlet port of the valve 6 is shut off by way of the right-hand side land (viewing FIG. 12), a part of working fluid of a high clutch pressure PC may be leaked via a slight annular aperture defined between the outer peripheral surface of the right-hand side land and the inner wall surface of the valve housing. As can be appreciated, the higher clutch pressure PC supplied into the inlet port of the valve 6, the greater the oil leakage to the drain port. In case of the clutch pressure control valve 4, when the duty-ratio controlled solenoid valve 5 is de-energized, the maximum increased external pilot pressure is supplied from the valve 5 to the pilot port of the clutch-pressure control valve 4. As appreciated from FIGS. 11 and 13, the external pilot pressure acts on the right-hand side land (viewing FIG. 13) in the same direction as the direction of spring bias and thus the spool of the valve 4 is maintained at its leftmost position, thereby reducing a throttling rate of the line-pressure inlet port of the valve 4 at the minimum by means of the left-hand side land. Therefore, with the duty-ratio controlled solenoid valve 5 de-activated, the working fluid of a pressure level essentially equal to the incoming line pressure is output from the outlet port of the valve 4 as a clutch pressure PC. As appreciated, when the clutch pressure PC is adjusted toward the maximum clutch pressure in accordance with the increase in the external pilot pressure produced by the duty-ratio controlled solenoid valve 5, a greatly increased pressure difference takes place between oil passages upstream of and downstream of the annular aperture defined between the outer peripheral surface of the right-hand side land (shutting off the drain port) and the inner wall surface of the valve housing. Undesired oil leakage via the annular aperture to the drain port may result from the above-noted great pressure difference. As is generally known, in modern four-wheel drive automobiles with a driving-torque control system as discussed previously, a hydraulic circuit for a transfer clutch is often communicated with a hydraulic circuit used for lubricating an automatic transmission, so as to effectively deliver a portion of superfluous working fluid to the lubricating system of the transmission. For example, as a result of regulation of the line pressure, superfluous working fluid is traditionally delivered to the lubricating system for use in the transmission. The previously-noted increase in oil leakage in the hydraulic circuit for the transfer clutch may result in a short supply of lubricating oil to be supplied to the lubricating system. Particularly in case of the four-wheel-drive low-speed range 4L, there is a tendency for the transmission to be loaded heavier in comparison with the two-wheel-drive high-speed range 2H. Therefore, when the four-wheel-drive low-speed range 4L is selected and thus the supply of the clutch pressure PC is stopped, the problem lacking lubricating oil is not negligible.